In recent years, with growing demand of higher function of electronics, high-density integration within electronic components and high-density mounting of electronic components have been developed. Printed wiring boards capable of high-density mounting and so on used for the electronic components have been developed in miniaturization and high density than ever before. In order to increase the density in the printed wiring boards, buildup multilayer printed wiring boards have been widely employed (for example, see Japanese Patent Application Laid-open (JP-A) No. H07-106767).
Buildup multilayer printed wiring boards are generally produced by laminating insulating layers and conducting circuit layers, the insulating layer having a thickness of 100 μm or less and being constituted with a resin composition. Examples of a method for connecting the conducting circuit layers are methods of forming via holes by a laser method, a photo method or the like, having replaced a drill process used conventionally. In these methods, via holes having small diameter are disposed on conducting circuit layers without restriction. Thereby, these methods can increase density in printed wiring board. Various interlayer insulation materials for buildup produced by these methods are proposed.
In order to increase density in printed wiring board, formation of microscopic circuit is required. A semi-additive process has been widely known as technique to increase the density. The semi-additive process includes the steps of: coating a surface of a circuit pattern of an inner layer circuit board with an insulating layer formed with an insulating resin; subjecting a surface of the insulating layer to roughening treatment; subjecting the surface to electroless plating treatment to form a base; protecting non-circuit forming parts by plating resist; subjecting circuit forming parts to electroplating to thickly deposit copper on the circuit forming parts; and performing resist removal and soft etching to form a conducting circuit on the insulating layer.
In such a semi-additive process, it is not preferable if concavity and convexity of a surface (degree of surface roughness) of the insulating layer are too large after the roughening treatment, since delay in speed of electric current passing a surface of copper, and decrease of hygroscopic solder heat resistance (solder heat resistance after applying hygroscopic environmental stress), may be caused. However, if the degree of surface roughness of the insulating layer is reduced, adhesion between plated metal and the insulating layer decreases. Thereby, defects such as expansion of plated metal, peeling in thermal shock tests including a cooling/heating cycle, and so on may be easily caused.